1. Field of the Invention
The present invention relates to a method of manufacturing a non-volatile memory cell, and more particularly, to a method of manufacturing a non-volatile memory cell having an oxide-nitride-oxide stack structure.
2. Description of the Prior Art
A flash memory mainly comprises a floating gate for storing electric charge, and a control gate disposed on the floating gate for controlling access of data, where the control gate is separated from the floating gate via a dielectric layer formed by an oxide-nitride-oxide (ONO) structure. Therefore, the memory can utilize a principle of thermal electrons or tunneling to store induced electric charge within the overlapped gates so as to store a signal “O” in the memory. If data stored in the memory needs to be changed, the process is to supply a small extra amount of energy in order to remove electrons stored in the floating gate so as to rewrite data.
The Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) structure is the most common structure of the non-volatile memory cell nowadays. The nitride silicon layer is a charge trapping medium of SONOS. Its floating gate is made by a punching effect or source side injection to save electrons in the floating gate as data. One shortcoming of SONOS is that the electron will punch the floating gate when the tunnel between the source and drain is too short. This becomes a problem with SONOS utilizes a smaller memory size. The other shortcoming of SONOS is a bad erasing effect that results in disordered data. This is why localized oxide-nitride-oxide memory is produced.
The localized oxide-nitride-oxide memory has a better erasing effect because of the localized oxide-nitride-oxide stack structure and the asymmetric structure. The prior localized oxide-nitride-oxide memory manufacture process has at least two photo masks. The first one is formed at the localized oxide-nitride-oxide memory and the other one is formed at the gate. The photo mask pattern becomes dense, however, causing the cost of manufacturing to increase. Because alignment accuracy is low and failure rate is high, the rework cost increases. The oxide-nitride-oxide stack structure is always inconsistent and the performance of the memory is not ideal.
Therefore, to develop a method of manufacturing the localized oxide-nitride-oxide memory to solve the above problem is very important.